Control units used for controlling in-vehicle devices are increasing for enhancing functions in a vehicle, in particular in an automobile. The control units are thereby required for coordinating with each other or sharing their data with each other.
The control units are therefore connected mutually through a communications line by respectively assembling circuits for data communications, and constitute a network, so called an in-vehicle local area network (in-vehicle LAN).
In this vehicular communications system using the network, if a certain control unit fails in functions, not only the certain control unit cannot operate a related device, but also other control units that exchange data with the certain control unit may not properly operate related devices.
This problem can be sometimes resolved by restarting the failing control unit. However, when the abnormal failing control unit is restarted, an in-vehicle battery is typically electrically opened, namely is detached from the vehicle. In this case, other normal control units are also initialized, so that recorded data that the normal control units store in a random access memory (RAM) may disappear simultaneously. This involves a long time in restoring the system to a normal state.